The present invention relates generally to the operation of pulverized fuel-fired steam generator furnaces and, more particularly, to a method of firing a pulverized fuel-fired steam generator furnace by conveying the fuel pulverized in the mill to the furnace entrained in a gaseous mixture of recirculated flue gas and air wherein the volume flow rate of the gaseous mixture is controlled in response to the feed rate of fuel to the mill and the volume flow rate of air in the gaseous mixture is controlled to maintain a desired oxygen level in the gaseous mixture entering the mill.
In firing solid fossil fuels, such as coal, lignite and peat, the fuel must be comminuted and dried before it can be introduced into the furnace. This is accomplished in the mill wherein the solid fuel is simultaneously pulverized and substantially dried. In order to obtain sufficient heat for adequate drying, a hot gaseous medium is admitted to the mill in a quantity necessary to provide sufficient heat to evaporate moisture in the fuel. The same gaseous medium is then used to transport the pulverized coal from the mill to the furnace for combustion therein.
Typically, the gaseous medium supplied to the mill for drying and subsequent transport of the pulverized fuel to the furnace is preheated air. As the pulverized fuel-air mixture enters the furnace and combustion begins, this oxygen is readily available to oxidize constituents in the pulverized fuel. One constituent of pulverized fuel is nitrogen bound in the complex organic structure of the fuel. This nitrogen tends to readily combine with oxygen in the earlier stages of combustion to form nitric oxide, a major pollutant. Thus there is concern that a high air to coal ratio in the transport stream could lead to increased formation of nitrogen oxide from fuel bound nitrogen.
Typically, the volume flow rate of preheated air to the mill is controlled in response to the feed rate of fuel to the mill. Also, the ratio of the flow rate of air to the feed rate of fuel changes over load with the ratio of air flow rate to the fuel feed rate increasing as load on the steam generator, and therefore fuel feed rate, decreases. As a consequence of the increase in the air flow rate to fuel feed rate as load decreases, the availability of oxygen in the pulverized fuel-air stream being conveyed to the furnace increases. That is, the air to fuel ratio, and therefore the pounds of oxygen available per pound of fuel, increases. This increase in air to fuel ratio as load decreases leads to a further increase in formation of nitrogen oxide from fuel bound nitrogen at low load.
One proposed solution to the problem of increased formation of nitrogen oxide at high air to fuel ratios in the transport stream has been to substitute recirculated flue gas for air as the gaseous medium feed to the mill to dry and transport the pulverized fuel. However, experience has shown that substituting recirculated flue gas for air as the transport medium led to ignition instability problems upon admission of the pulverized fuel-recirculated flue gas mixture to the furnace.
It is therefore an object of the present invention to provide a method of firing a pulverized fuel-steam generator wherein a mixture of recirculated flue gas and air can be used as the drying and conveying medium in the mill while ensuring satisfactory ignition stability.
A further object of the present invention is to limit nitric oxide formation from fuel bound nitrogen via controlling oxygen availability in the transport medium.
Still another object of the present invention is to maintain efficient drying within the mill while using a recirculated fuel gas and air mixture as the drying medium.